The present invention relates to a solid-state image pickup device suitable for a CCD solid-state image pickup device, typically, of a total pixel readout type, and a fabrication method thereof.
FIG. 17 shows an essential portion of an image pickup region of a conventional total pixel readout type CCD solid-state image pickup device. A CCD solid-state image pickup device 1 shown in FIG. 17, if being of an inter line transfer (IT) type, includes an image pickup region 4 and a horizontal transfer register having a CCD structure (not shown). The image pickup region 4 includes a plurality of light receiving portions 2 for photoelectric conversion, which portions are taken as pixels arranged in a matrix, and a plurality of vertical transfer registers 3 each of which has a CCD structure and which is formed on one side of each column of the light receiving portions 2. The horizontal transfer register is used for transferring signal charges transferred from the vertical transfer registers 3 to an output unit.
FIG. 18A is a sectional view taken on line A1xe2x80x94A1 of FIG. 17, and FIG. 18B is a sectional view taken on line B1xe2x80x94B1 of FIG. 17. Referring to FIGS. 18A and 18B, the vertical transfer register 3 has transfer electrodes having a three-layer structure formed on a transfer channel region of a silicon semiconductor base 6 via a gate insulating film 7. These transfer electrodes are composed of first transfer electrodes 8A formed by a first polysilicon layer, second transfer electrodes 8B formed by a second polysilicon layer, and third transfer electrodes 8C formed by a third polysilicon layer, which are repeatedly arranged along a charge transfer direction xe2x80x9caxe2x80x9d. Reference numeral 9 designates an interlayer insulating film. Each of the first transfer electrodes 8A is formed into a band-shape extending in the horizontal direction in such a manner as to be common to a plurality of columns of the vertical transfer registers 3. The same is true for the second and third transfer electrodes 8B and 8c. 
In the region between the light receiving portions 2 adjacent to each other in the vertical direction, the first, second, and third transfer electrodes 8A, 8B and 8C are sequentially stacked.
The solid-state image pickup device 1 is configured such that the transfer electrode 8 of the vertical transfer register 3 is divided into the three parts, that is, the first, second and third transfer electrodes 8A, 8B and 8C for each pixel (light receiving portion 2), and is three-phase driven for total pixel readout by applying three-phase vertical drive pulses xc3x8V1, xc3x8V2, and xc3x8V3 shown in FIG. 19 to these transfer electrodes 8A, 8B and 8C, respectively.
Another CCD solid-state image pickup device 11 having a configuration shown in FIG. 15 has been proposed. The device 11 is four-phase driven for total pixel readout by applying four-phase vertical drive pulses to transfer electrodes having a three-layer structure of each vertical transfer register.
FIG. 16A is a sectional view taken on line A2xe2x80x94A2 of FIG. 15. Referring to FIGS. 15 and 16A, the CCD solid-state image pickup device 11 is configured such that transfer electrodes 8 of a vertical transfer register 3 are formed by three polysilicon layers. To be more specific, second and fourth transfer electrodes 8B and 8D formed by the second polysilicon layer are alternately arranged along a charge transfer direction; each first transfer electrode 8A formed by the first polysilicon layer is disposed between the second and fourth transfer electrodes 8B and 8D arranged in this order, for example, from the left side in FIG. 16A; and each third transfer electrode 8C formed by the third polysilicon layer is disposed between the fourth and second transfer electrodes 8D and 8B arranged in this order, for example, from the left side in FIG. 16A.
FIG. 16B is a sectional view taken on line B2xe2x80x94B2 of FIG. 15. Referring to FIG. 16B, in the region between the light receiving portions 2 adjacent to each other in the vertical direction, the second and fourth transfer electrodes 8B and 8D formed by the second layer are stacked on the first transfer electrode 8A formed by the first layer, and the third transfer electrode 8C formed by the third layer is stacked on the second and fourth electrodes 8B and 8D.
The solid-state image pickup device 11 is configured such that the transfer electrode 8 of the vertical transfer register 3 is divided into the four parts, that is, the first, second, third, and fourth transfer electrodes 8A, 8B, 8C and 8D for each pixel (light receiving portion 2), and is four-phase driven for total pixel readout by applying four-phase vertical drive pulses xc3x8V1, xc3x8V2, xc3x8V3, and xc3x8V4, shown in FIG. 3 to these transfer electrodes 8A to 8D, respectively.
The other configuration is the same as that shown in FIG. 17 and FIGS. 18A and 18B, and therefore, corresponding parts are designated by the same characters and the overlapped explanation is omitted.
In the CCD solid-state image pickup device 1 shown in FIG. 17, since the vertical transfer register 3 is three-phase driven by the transfer electrode 8 divided into the three parts, that is, the first, second and third transfer electrodes 8A, 8B and 8C, the accumulated charge capacity in the vertical transfer register 3 is equivalent to one-third of the accumulated charge capacity in the vertical transfer path for one pixel. As a result, to ensure a sufficient accumulated charge capacity in the transfer portion, the width W1 of the transfer path must be broadened; however, if the width W1 of the transfer path is broadened, the area of the light receiving portion 2 is reduced in proportional to the broadened width W1.
The areas of the three transfer electrodes 8A to 8C divided from the transfer electrode 8 for each pixel may be desirable to be equalized to each other for ensuring a larger accumulated charge capacity; however, they actually become uneven largely depending on variations in processed line width among the transfer electrodes 8A to 8C. As a result, the accumulated charge capacity is determined by one of the transfer electrodes 8A to 8C having the smallest area, to thereby reduce the actual charge amount.
In the CCD solid-state image pickup device 11 shown in FIG. 15, which is four-phase driven for total pixel readout by the three-layer electrode structure, since the accumulated charge capacity is equivalent to two-fourth of the accumulated charge capacity in the vertical transfer path for one pixel, it becomes larger than that in the CCD solid-state image pickup device 1 shown in FIG. 17, which is three-phase driven for total pixel readout by the three-layer electrode structure.
The CCD solid-state image pickup device 11, however, has the following disadvantage: namely, a variation in line width occurs between the transfer electrode 8A formed by the first layer and each of the transfer electrodes 8B and 8D formed by the second layer and also a misalignment occurs between the transfer electrode 8A formed by the first layer and each of the transfer electrodes 8B and 8D formed by the second layer, so that variations occur among lengths L1, L2, L3 and L4 of the two-phase transfer regions each of which is composed of the adjacent transfer electrodes for two-phases and is taken as a factor determining the accumulated charge capacity, to thereby reduce the actual charge amount.
An object of the present invention is to provide a solid-state image pickup device intended to increase the accumulated charge capacity, increase the area of a light receiving portion, and prevent a reduction in actual charge amount due to variations in processed dimension between transfer electrodes, and a method of fabricating the solid-state image pickup device.
To achieve the above object, according to a first aspect of the present invention, there is provided a solid-state image pickup device including: a plurality of light receiving portions arranged in a matrix; and a vertical transfer register which is four-phase driven by transfer electrodes of a three-layer structure, said vertical transfer register being provided for each of columns of said light receiving portions; wherein those, formed by the first layer, of said transfer electrodes are composed of two kinds of transfer electrodes alternately arranged in a charge transfer direction.
With this configuration, of the transfer electrodes of the vertical transfer register which is four-phase driven, the first and third transfer electrodes formed by the first layer are alternately arranged; the second transfer electrodes formed by the second layer are each arranged between the first and third transfer electrodes in such a manner as to be laid across them; and the fourth transfer electrodes formed by the third layer are each arranged between the third and first transfer electrodes in such a manner as to be laid across them. As a result, even if there occur variations in processed dimension between the first and third transfer electrodes of the first layer, the length of the two-phase transfer region composed of the first or third transfer electrode of the first layer and the second transfer electrode of the second layer is usually equalized to the length of the two-phase transfer region composed of the first or third transfer electrode of the first layer and the fourth transfer electrode of the third layer. Accordingly, it is possible to increase the accumulated charge capacity in the vertical transfer register, and hence to prevent the reduction in actual charge amount.
Further, since the vertical transfer register which is four-phase driven is provided, the accumulated charge capacity is equivalent to two-fourth of the accumulated charge capacity in the vertical transfer path per one pixel. This makes it possible to make the width of the vertical transfer path thinner and hence to make the area of the light receiving portion wider.
According to a second aspect of the present invention, there is provided a solid-state image pickup device including: a plurality of light receiving portions arranged in a matrix; and a vertical transfer register which is four-phase driven by first and third transfer electrodes formed by a first layer and second and fourth electrodes formed by a second layer which are alternately arranged in the order of said first, second, third and fourth transfer electrodes, said vertical transfer register being provided for each of columns of said light receiving portions; wherein one of said second and fourth transfer electrodes of the second layer is formed independently for each of said vertical transfer registers; and said one of said second and fourth transfer electrodes, which is formed independently for each of said vertical transfer registers, is connected to an interconnection formed by a third layer.
With this configuration, since the thickness of each interlayer insulating film between the adjacent two of all the transfer electrodes is determined by the thickness of the interlayer insulating film formed on the surfaces of the first and third transfer electrode of the first layer, and therefore, it is equalized and accordingly, it is possible to prevent occurrence of the potential dip upon charge transfer.
With this configuration, of the transfer electrodes of the vertical transfer register which is four-phase driven, the first and third transfer electrodes of the first layer are alternately arranged; and the second and fourth transfer electrodes of the second layer are respectively arranged between the first and third transfer electrodes and between the third and first electrodes of the first layer in such a manner as to be laid across them. As a result, even if there occur variations in processed dimension between the first and third transfer electrodes of the first layer, the length of the two-phase transfer region composed of the first or third transfer electrode of the first layer and the second or fourth transfer electrode of the second layer is usually equalized. Accordingly, it is possible to increase the accumulated charge capacity in the vertical transfer register, and hence to prevent the reduction in actual charge amount.
Further, since the vertical transfer register which is four-phase driven is provided, the accumulated charge capacity is equivalent to two-fourth of the accumulated charge capacity in the vertical transfer path per one pixel. This makes it possible to make the width of the vertical transfer path thinner and hence to make the area of the light receiving portion wider.
According to a third aspect of the present invention, there is provided a method of fabricating a solid-state image pickup device, including the steps of: forming, mask patterns corresponding to patterns of first and third transfer electrodes which are to be alternately arranged in each vertical transfer register formation region and which are to extend in parallel to each other between light receiving portions adjacent to each other in the vertical direction, on a first electrode material layer; forming side walls on each of said mask patterns; patterning said first electrode material layer via said mask patterns having said side walls, to form first and third transfer electrodes formed by the first layer; forming second transfer electrodes by a second electrode material layer via an insulating film in such a manner that each of said second transfer electrodes is disposed between said first and third transfer electrodes of the first layer arranged in this order in said vertical transfer register formation region and between said light receiving portions; and forming fourth transfer electrodes by a third electrode material layer via an insulating film in such a manner that each of said fourth transfer electrodes between said third and first transfer electrodes of the first layer arranged in this order in said vertical transfer register formation region and between said light receiving portions.
With this configuration, since the first and third electrodes formed by the first layer are alternately arranged and then the second electrodes formed by the second layer are each disposed between the first and third electrodes and the fourth electrodes formed by the third layer are each disposed between the third and first electrodes, even if there occur variations in processed dimension between the first and third electrodes of the first layer, it is possible to usually keep the length of each two-phase transfer region composed of the adjacent transfer electrodes for two-phases.
Further, since the first electrode material layer is patterned via the mask patterns having the side walls, the gap between the first and third electrodes of the first layer extending in parallel to each other between the light receiving portions adjacent to each other in the vertical direction is narrower than the minimum line width of the photolithography. Accordingly, it is possible to form the four-phase transfer electrodes having a sufficient width in the narrow region between the light receiving portions.
According to a fourth aspect of the present invention, there is provided a method of fabricating a solid-state image pickup device, including the steps of: forming first and third electrodes by a first electrode material layer in such a manner that said first and third electrodes are alternately arranged in each vertical transfer register formation region and the adjacent two of said first and third electrodes extend in parallel to each other between light receiving portions adjacent to each other in the vertical direction; forming an interlayer insulating film on the surfaces of said first and third transfer electrodes of the first layer; forming second transfer electrodes by a second electrode material layer in such a manner that each of said second transfer electrodes is disposed between said first and third electrodes of the first layer arranged in this order and extends between said light receiving portions; forming fourth transfer electrodes by said second electrode material layer in such a manner that each of said fourth transfer electrodes is disposed between said third and first transfer electrode of the first layer arranged in this order independently only in each of said vertical transfer register formation regions; and forming interconnections by a third conductive material layer in such a manner that each of said interconnections extends between said light receiving portions to be connected to said independent fourth transfer electrode of the second layer.
With this configuration, after the first and second transfer electrodes of the first layer are formed, the interlayer insulating film is formed on the surfaces of the first and second transfer electrodes and then the second and fourth electrodes of the second layer are formed between the first and third transfer electrodes and between the third and first electrodes, and accordingly, each interlayer insulating film between the adjacent two of the all the transfer electrodes is equalized.
Further, since the first and third electrodes formed by the first layer are alternately arranged and then the second electrodes formed by the second layer are each disposed between the first and third electrodes and the fourth electrodes formed by the third layer are each disposed between the third and first electrodes, even if there occur variations in processed dimension between the first and third electrodes of the first layer, it is possible to usually keep the length of each two-phase transfer region composed of the adjacent transfer electrodes for two-phases.